In natural mineral oil deposits, mineral oil is present in the cavities of porous reservoir rocks which are sealed toward the surface of the earth by impermeable top layers. The cavities may be very fine cavities, capillaries, pores or the like. Fine pore necks may, for example, have a diameter of only approx. 1 μm. As well as mineral oil, including fractions of natural gas, a deposit also comprises water with a greater or lesser salt content.
In mineral oil production, a distinction is drawn between primary, secondary and tertiary production.
In primary production, after commencement of drilling of the deposit, the mineral oil flows of its own accord through the borehole to the surface owing to the autogenous pressure of the deposit. The autogenous pressure can be caused, for example, by gases present in the deposit, such as methane, ethane or propane. The autogenous pressure of the deposit, however, generally declines relatively rapidly on extraction of mineral oil, such that usually only approx. 5 to 10% of the amount of mineral oil present in the deposit, according to the deposit type, can be produced by means of primary production. Thereafter, the autogenous pressure is no longer sufficient to produce mineral oil.
After primary production, secondary production is therefore typically used. In secondary production, in addition to the boreholes which serve for the production of the mineral oil, known as the production boreholes, further boreholes are drilled into the mineral oil-bearing formation. These are known as injection boreholes, through which water is injected into the deposit (known as “water flooding”), in order to maintain the pressure or to increase it again. As a result of the injection of the water, the mineral oil is gradually forced through the cavities in the formation, proceeding from the injection borehole, in the direction of the production borehole. However, this works only for as long as the cavities are completely filled with oil and the more viscous oil is pushed onward by the water. As soon as the mobile water breaks through cavities, it flows on the path of least resistance from this time onward, i.e. through the channel formed, and no longer pushes the oil onward. By means of primary and secondary production, therefore, generally only approx. 30 to 35% of the amount of mineral oil present in the deposit can be produced.
After the measures of secondary mineral oil production, measures of tertiary mineral oil production (also known as “Enhanced Oil Recovery (EOR)”) are therefore also used to further enhance the oil yield. This includes processes in which suitable chemicals, such as surfactants and/or polymers, are used as assistants for oil production. An overview of tertiary oil production using chemicals can be found, for example, in the article by D. G. Kessel, Journal of Petroleum Science and Engineering, 2 (1989) 81-101.
The techniques of tertiary mineral oil production include what is known as “polymer flooding”. Polymer flooding involves injecting an aqueous solution of a thickening polymer through the injection boreholes into the mineral oil deposit, the viscosity of the aqueous polymer solution being matched to the viscosity of the mineral oil. As a result of the injection of the polymer solution, the mineral oil, as in the case of water flooding, is forced through the cavities mentioned in the formation, proceeding from the injection borehole, in the direction of the production borehole, and the mineral oil is produced through the production borehole. By virtue of the fact that the polymer formulation, however, has about the same viscosity as the mineral oil, the risk is reduced that the polymer formulation breaks through to the production borehole with no effect, and hence the mineral oil is mobilized much more homogeneously than in the case of use of mobile water. It is thus possible to mobilize additional mineral oil in the formation. Details of polymer flooding and of polymers suitable for this purpose are disclosed, for example, in “Petroleum, Enhanced Oil Recovery, Kirk-Othmer, Encyclopedia of Chemical Technology, online edition, John Wiley & Sons, 2010”.
For polymer flooding, a multitude of different thickening polymers have been proposed, especially high molecular weight polyacrylamide, copolymers of acrylamide and further comonomers, for example vinylsulfonic acid or acrylic acid. Polyacrylamide may especially be partly hydrolyzed polyacrylamide, in which some of the acrylamide units have been hydrolyzed to acrylic acid. In addition, it is also possible to use naturally occurring polymers, for example xanthan or polyglycosylglucan, as described, for example, by U.S. Pat. No. 6,392,596 B1 or CA 832 277.
Also known is the use of hydrophobically associating copolymers for polymer flooding. These are understood by the person skilled in the art to mean water-soluble polymers which have lateral or terminal hydrophobic groups, for example relatively long alkyl chains. In aqueous medium, such hydrophobic groups can associate with themselves or with other substances having hydrophobic groups. This forms an associative network by which the medium is thickened. Details of the use of hydrophobically associating copolymers for tertiary mineral oil production are described, for example, in the review article by Taylor, K. C. and Nasr-El-Din, H. A. in J. Petr. Sci. Eng. 1998, 19, 265-280.
EP 705 854 A1, DE 100 37 629 A1 and DE 10 2004 032 304 A1 disclose water-soluble, hydrophobically associating copolymers and the use thereof, for example in the construction chemistry sector. The copolymers described comprise acidic monomers, for example acrylic acid, vinylsulfonic acid, acrylamidomethylpropanesulfonic acid, basic monomers such as acrylamide, dimethylacrylamide, or monomers comprising cationic groups, for example monomers having ammonium groups, and also monomers which can bring about the hydrophobic association of the individual polymer chains.
Our prior application WO 2010/133527 A2 discloses hydrophobically associating copolymers which comprise at least hydrophilic, monoethylenically unsaturated monomers, for example acrylamide, and monoethylenically unsaturated, hydrophobically associating monomers. The hydrophobically associating monomers have a block structure and have—in this sequence—an ethylenically unsaturated group, optionally a linking group, a first polyoxyalkylene block which comprises at least 50 mol % of ethyleneoxy groups, and a second polyoxyalkylene group which consists of alkyleneoxy groups having at least 4 carbon atoms. The application discloses the use of such copolymers as thickeners, for example for polymer flooding, for construction chemical applications or for detergent formulations.
Our prior application WO 2011/015520 A1 discloses a process for preparing hydrophobically associating copolymers by polymerizing water-soluble, monoethylenically unsaturated surface-active monomers and monoethylenically unsaturated hydrophilic monomers in the presence of surfactants, and the use of such copolymers for polymer flooding.
For polymer flooding, an aqueous polymer solution is injected through a borehole (called the injection borehole) into a mineral oil deposit, and the viscosity of the polymer solution under formation conditions should correspond approximately to the viscosity of the mineral oil. Suitable polymers for polymer flooding must therefore also have the thickening action under the conditions of the mineral oil deposit, i.e. at temperatures above room temperature and in the presence of formation water with a high salt content. Formation waters may in the extreme case comprise up to 35% by weight of salts. The salts are, for example, alkali metal salts, but also alkaline earth metal salts. Formation temperatures may be up to 150° C.
Studies on partly hydrolyzed polyacrylamide and copolymers of acrylamide and acrylamide-methylpropanesulfonic acid show that the salt tolerance of the polymers can be enhanced by the incorporation of sulfo groups (see, for example, Masoud Rashidi, Anne Marit Blokhus, Arne Skauge, Journal of Applied Polymer Science, Vol. 117 (2010), pages 1551-1557). In the case of such polymers, however, the viscosity decreases with increasing temperature. Thus, to achieve a viscosity sufficient for polymer flooding, higher amounts of polymer have to be used, which impairs the economic viability of polymer flooding.